Cytochalasin D inhibits the polymerization of actin, raises the critical concentration of monomeric actin, and greatly increases the ATPase activity of actin. When added to polymerized actin, cytochalasin D causes rapid depolymerization to a new steady state level. Spectrin/actin complex from red blood cells nucleates and accelerates the polymerization of actin and lowers the critical concentration of monomeric actin at steady state. Cytochalasin D inhibits polymerization of actin nucleated by spectrin/actin complex but does not depolymerize the actin. These observations support a model of actin polymerization in which the filament elongates at one end more rapidly than at the other. At steady state, actin monomers are continually adding to one end of the filament and leaving the other end. Cytochalasin D, according to this model, would inhibit addition of monomers at the net polymerizing end, perhaps by uncoupling the ATPase reaction, and spectrin/actin complex would block loss of monomers at the net depolymerizing end. These experiments also lead to a picture of the spectrin/actin complex as decameric oligomers of actin crosslinked by spectrin tetramers. In other experiments we have found that profilin forms a weak 1:1 molar complex with actin and enhances the rate of exchange of actin bound ATP.